The influence of light on mood and emotion
In the 7 th century William Shakespeare wrote "A sad tale's best for winter". However, he was not the first who understood the power of light on our psyche. 2000 years ago Hippocrates, the father of modern medicine, already acknowledged that the absence of light, particularly in winter, can produce diseases. Since then, the impact of light on mood and the use of bright light as a treatment-option for affective disorders have been studied extensively by scientists (for a review see Terman & Terman 2005). Light is the major zeitgeber for human circadian rhythms, much more powerful than social zeitgebers eg. work or school schedules (Challet 2007). Non-visual effects of light include hormone regulation, the synchronization of the circadian system, the regulation of body temperature, but also the regulation of cognition and alertness (Brainard et al. 2005; Lockley et al. 2006; Vandewalle et al. 2009; Dijket al. 2009; Cajochen et al. 2007). Bright light treatments are dating back to Lewy et al. (1987) who could demonstrate that exposure to bright white light (a mixed spectrum of wavelengths similar to day light) can adjust circadian rhythms and suppress melatonin. The most extensive clinical trials on bright light therapy have focused on seasonal affective disorders suggesting that light can modulate mood in the long term (Wirz-Justice et al. 2004). Today light therapy is used to treat different disorders like sleep disorders, affective disorders, dementia etc. (for a review see Shirani et al. 2009). This chapter will give an overview about the neurobiological basis for light therapy and discuss different mood disorders responsive to light therapy. Additionally, the influence of light on normal brain emotional processing will be discussed.